So, I honestly fell in love with this whole concept and all of your drawings which got me excited to join in the fun! If you don't mind. I haven't drawn much in a while but this is what I got. I recently finish the last episode of Anohana and I decided to draw Menma!

I used to have a lopsiding problem, but then I discovered it was actually because of my paper position! I had it flat down on my desk, when I should have had it propped up on something to see it all in the proper perspective. If you just lay your paper flat, try propping it up at an angle instead.

i have a similar problem, but its because i start turning my head sideways as i get more comfortable. you may notice in a lot of my drawings they are almost perfectly rotated 10 or 15 degrees every time. i have to pay serious attention to my guidelines to make sure not only they are straight (parallel/perpendicular) but also that i follow them properly.

yes paper position/angle can also affect this. some people draw with their head on the side looking at their paper from the side as well, which can skew a drawing as well. if possible try drawing with a drawing board or something that makes the paper more upright so you dont lean over your work.